184 research outputs found
The Surface Chemical Composition of Lunar Samples and Its Significance for Optical Properties
The surface iron, titanium, calcium, and silicon concentration in numerous lunar soil and rock samples was determined by Auger electron spectroscopy. All soil samples show a large increase in the iron to oxygen ratio compared with samples of pulverized rock or with results of the bulk chemical analysis. A solar wind simulation experiment using 2 keV energy alpha -particles showed that an ion dose corresponding to approximately 30,000 years of solar wind increased the iron concentration on the surface of the pulverized Apollo 14 rock sample 14310 to the concentration measured in the Apollo 14 soil sample 14163, and the albedo of the pulverized rock decreased from 0.36 to 0.07. The low albedo of the lunar soil is related to the iron + titanium concentration on its surface. A solar wind sputter reduction mechanism is discussed as a possible cause for both the surface chemical and optical properties of the soil
The search for the cause of the low albedo of the moon
Experimentation concerning lunar weathering and its effect on the albedo of the surface cover consisted of: (1) determination of the surface chemical composition of lunar soil and ground-up rock samples by Auger electron spectroscopy, (2) measurement of the optical albedo of these samples, and (3) proton or alpha-particle irradiation of terrestrial rock chips and rock powders and of ground-up lunar rock samples in order to determine the optical and surface chemical effect of simulated solar wind
Electrical properties of Apollo 17 rock and soil samples and a summary of the electrical properties of lunar material at 450 MHz frequency
The dielectric constant and the voltage absorption length was measured for four Apollo 17 soil samples (73241, 74220, 75061, 76501) and for two Apollo 17 rock samples (76315 and 79135) at 450 MHz frequency. The dielectric constant and absorption length measurements made on the lunar samples are reviewed and related to the transition element concentration in these samples. The significance of the laboratory measurements for radar observations is discussed
Comparison of |Q|=1 and |Q|=2 gauge-field configurations on the lattice four-torus
It is known that exactly self-dual gauge-field configurations with
topological charge |Q|=1 cannot exist on the untwisted continuum 4-torus. We
explore the manifestation of this remarkable fact on the lattice 4-torus for
SU(3) using advanced techniques for controlling lattice discretization errors,
extending earlier work of De Forcrand et. al. for SU(2). We identify three
distinct signals for the instability of |Q|=1 configurations, and show that
these manifest themselves early in the cooling process, long before the
would-be instanton has shrunk to a size comparable to the lattice
discretization threshold. These signals do not appear for our |Q|=2
configurations. This indicates that these signals reflect the truly global
nature of the instability, rather than local discretization effects.
Monte-Carlo generated SU(3) gauge field configurations are cooled to the
self-dual limit using an O(a^4)-improved gauge action chosen to have small but
positive O(a^6) errors. This choice prevents lattice discretization errors from
destroying instantons provided their size exceeds the dislocation threshold of
the cooling algorithm. Lattice discretization errors are evaluated by comparing
the O(a^4)-improved gauge-field action with an O(a^4)-improved action
constructed from the square of an O(a^4)-improved lattice field-strength
tensor, thus having different O(a^6) discretization errors. The number of
action-density peaks, the instanton size and the topological charge of
configurations is monitored. We observe a fluctuation in the total topological
charge of |Q|=1 configurations, and demonstrate that the onset of this unusual
behavior corresponds with the disappearance of multiple-peaks in the action
density. At the same time discretization errors are minimal.Comment: 12 pages, 9 figures, submitted to Phys. Rev.
Using digital and hand printing techniques to compensate for loss: re-establishing colour and texture in historic textiles
Conservators use a range of 'gap filling' techniques to improve the structural stability and presentation of objects. Textile conservators often use fabric supports to provide reinforcement for weak areas of a textile and to provide a visual infill in missing areas. The most common technique is to use dyed fabrics of a single colour but while a plain dyed support provides good reinforcement, it can be visually obtrusive when used with patterned or textured textiles. Two recent postgraduate dissertation projects at the Textile Conservation Centre (TCC) have experimented with hand printing and digital imaging techniques to alter the appearance of support fabrics so that they are less visually obtrusive and blend well with the colour and texture of the textile being supported. Case studies demonstrate the successful use of these techniques on a painted hessian rocking horse and a knitted glove from an archaeological context
Spin-3/2 Nucleon and Delta Baryons in Lattice QCD
We present first results for masses of spin-3/2 N and Delta baryons in
lattice QCD using Fat-Link Irrelevant Clover (FLIC) fermions. Spin-3/2
interpolating fields providing overlap with both spin-3/2 and spin-1/2 states
are considered. In the isospin-1/2 sector, we observe, after appropriate spin
and parity projection, a strong signal for the J^P=3/2^- state together with a
weak but discernible signal for the 3/2^+ state with a mass splitting near that
observed experimentally. We also find good agreement between the 1/2^+/- masses
and earlier nucleon mass simulations with the standard spin-1/2 interpolating
field. For the isospin-3/2 Delta states, clear mass splittings are observed
between the various 1/2^+/- and 3/2^+/- channels, with the calculated level
orderings in good agreement with those observed empirically.Comment: 17 pages, 8 figures, 2 table
Quantum gravity and the standard model
We show that a class of background independent models of quantum spacetime
have local excitations that can be mapped to the first generation fermions of
the standard model of particle physics. These states propagate coherently as
they can be shown to be noiseless subsystems of the microscopic quantum
dynamics. These are identified in terms of certain patterns of braiding of
graphs, thus giving a quantum gravitational foundation for the topological
preon model proposed by one of us.
These results apply to a large class of theories in which the Hilbert space
has a basis of states given by ribbon graphs embedded in a three-dimensional
manifold up to diffeomorphisms, and the dynamics is given by local moves on the
graphs, such as arise in the representation theory of quantum groups. For such
models, matter appears to be already included in the microscopic kinematics and
dynamics.Comment: 12 pages, 21 figures, improved presentation, results unchange
Calorons with non-trivial holonomy on and off the lattice
We discuss recent solutions for SU(2) calorons with non-trivial holonomy at
higher charge, both through analytic means and using cooling, as well as
extensive lattice studies for SU(3).Comment: 12 pages, 16 figures in 34 parts, 4 talks presented at Lattice
2004(topology
Topological Structure of the QCD Vacuum Revealed by Overlap Fermions
Overlap fermions preserve a remnant of chiral symmetry on the lattice. They
are a powerful tool to investigate the topological structure of the vacuum of
Yang-Mills theory and full QCD. Recent results concerning the localization of
topological charge and the localization and local chirality of the overlap
eigenmodes are reported. The charge distribution is radically different, if a
spectral cut-off for the Dirac eigenmodes is applied. The density q(x) is
changing from the scale-a charge density (with full lattice resolution) to the
ultraviolet filtered charge density. The scale-a density, computed on the Linux
cluster of LRZ, has a singular, sign-coherent global structure of co-dimension
1 first described by the Kentucky group. We stress, however, the cluster
properties of the UV filtered topological density resembling the instanton
picture. The spectral cut-off can be mapped to a bosonic smearing procedure.
The UV filtered field strength reveals a high degree of (anti)selfduality at
"hot spots" of the action. The fermionic eigenmodes show a high degree of local
chirality. The lowest modes are seen to be localized in low-dimensional
space-time regions.Comment: 13 pages, 11 figures, accepted to appear in the Proceedings of "HLRB,
KONWIHR and Linux-Cluster: Review, Results and Future Projects Workshop",
Leibniz Rechenzentrum Munich, December 200
Excited Baryons in Lattice QCD
We present first results for the masses of positive and negative parity
excited baryons calculated in lattice QCD using an O(a^2)-improved gluon action
and a fat-link irrelevant clover (FLIC) fermion action in which only the
irrelevant operators are constructed with APE-smeared links. The results are in
agreement with earlier calculations of N^* resonances using improved actions
and exhibit a clear mass splitting between the nucleon and its chiral partner.
An correlation matrix analysis reveals two low-lying J^P=(1/2)^- states with a
small mass splitting. The study of different Lambda interpolating fields
suggests a similar splitting between the lowest two Lambda1/2^- octet states.
However, the empirical mass suppression of the Lambda^*(1405) is not evident in
these quenched QCD simulations, suggesting a potentially important role for the
meson cloud of the Lambda^*(1405) and/or a need for more exotic interpolating
fields.Comment: Correlation matrix analysis performed. Increased to 400
configurations. 22 pages, 13 figures, 15 table
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